My friend put a 4 inch treated wood gate post in the ground and we filled the hole with concrete. A neighbor built a fence around the same time, using similar-looking posts. Six years later, the post rotted out but the fence is still going strong. Did we do something wrong?
The most common failure with posts in concrete is "collar rot", where the post rots right at the point where it exits the concrete at the ground line. This can be easily avoided if the concrete is slightly above ground grade and domed or tapered away from the post so water doesn't lay against the wood. Drainage is the key, keep standing water away from the posts and they will last a lot longer.
This all depends on soil conditions.
Loamy soil with good drainage, the concrete is probably OK. Clay soil, the concrete doesn't really do anything short of trapping water against the wood.
In both cases, many people suggest not using concrete at all. Instead, dig the hole deeper than the post and then add some gravel to the bottom. Insert the post, then tamp loose gravel/crushed rock around the post. This gives a sturdy post, with plenty of drainage so that water is never trapped against the post.
I was lazy and went the concrete route, but did a somewhat half-and-half. I dug the hole, added gravel, put in the post, added a bit more gravel, then quick-set concrete. It was quicker and less labor, but I still got the advantage of some drainage at the bottom.
Ideally, you'd never put any wood underground, but that's not always a practical solution in terms of cost/labor. If that's not an issue, consider some of the newer synthetic woods on the market or even galvanized posts.
Finally, some treated lumber isn't just treated very well. Maybe there was a bad batch. Also, be sure not to cut the post and put the cut-side down. If you do do that, you need to dip the post in some form of rot preventer first (basically, re-treating the exposed wood)
Final comment: If I ever build a fence again, I won't bother with PT posts. I'd spring for Cedar and then treat the bottom portions myself. The PT stuff was so wet it all ended up twisting and splitting for years afterward.